The present invention relates to a device which enables the direct monitoring of the absolute velocity of points of a driven pile during the impact between a hammer and the pile head.
The usefulness of knowing the velocity pulses during impact is well known to engineers, who use such pulses to obtain indications on pile load capacity, distribution of side friction along the pile, driving efficiency, etc.
At present, velocity pulses are determined indirectly, by using accelerometers rigidly attached to pile walls or to the pile tip. The accelerations are then integrated with respect to time to obtain velocities. The integration, simple in principle, creates in practice serious problems, so that very seldom are accurate velocity measurements obtainable, unless sophisticated and costly equipment, together with highly trained technicians, are employed.
On the other hand, the direct measurement of velocities of pile points, for instance with velocity transducers, would require that a fixed reference point be available, for measuring velocities with respect to it. Such a fixed reference point is normally unavailable, as the soil surface around the pile is kept on vibrating by the impacts, which occur about one per second. Moreover, even if the soil surface was vibration-free, a velocity measuring device should be reset very frequently, i.e. each time the increment of pile penetration equals the stroke of the impacting device.
Similar inconveniences, often more severe, are found when it is attempted to obtain velocity as a derivative of the displacement with respect to time.
In seismic and geophysical measurements, devices called geophones are used. The geophones are basically core and coil transducers where one of the two sliding elements is connected to a spring-supported mass, which is considered stationary as a first approximation. The geophones, however, are not suitable for measuring velocities of points of driven piles, where net displacements are much larger than seismic ones; moreover, in the case of driven piles, the spring supported mass would be vibrating without control.